Discovery and Development of 8-Substituted Cycloberberine Derivatives as Novel Antibacterial Agents against MRSA

8-Acetoxycycloberberine (2) with a unique skeleton was first identified to display a potent activity profile against Gram-positive bacteria, especially methicillin-resistant S. aureus (MRSA) with minimum inhibitory concentration (MIC) values of 1-8 μg/mL, suggesting a possible novel mechanism of action against bacteria. Taking 2 as the lead, 23 new 8-substituted cycloberberine (CBBR) derivatives including ether, amine, and amide were synthesized and evaluated for their antibacterial effect. The structure-activity relationship revealed that the introduction of a suitable substituent at the 8-position could greatly enhance the potency against MRSA. Among them, compounds 5d and 9e demonstrated equally effective anti-MRSA potency as lead 2, with an advantage of having a more stable pharmacokinetics feature. A preliminary mechanism study indicated that compound 9e acted upon bacteria partly through catalyzing the cleavage of bacterial DNA. Therefore, we consider that 8-substituted CBBR derivatives constitute a promising class of antibacterial agents in the treatment of MRSA infections.